Studies in other laboratories have shown that specific areas of rat brain are damaged following repeated exposures to oxygen at high pressure (OHP). The map of OHP damaged areas closely matches our map of iron-rich areas of the brain. Using the cupric-silver method to detect neuronal degeneration, we examined animals which had been exposed daily to OHP for 1-5 days to determine the earliest appearance of degeneration in the various brain areas. Areas most severely affected after 1 or 2 exposures were the auditory nuclei. These nuclei contain relatively moderate amounts of iron but have a high metabolic rate. Other areas with high metabolic rate but with no iron displayed no damage under OHP. Areas with the highest amount of iron-globus pallidus and substantia nigra were barely affected. It is concluded at this stage of the study that damage to areas of the brain is a function of both iron content and rate of metabolism.